1. Field of the Invention
This invention relates to electrophotography in which electrostatic latent images are visualized by charged colored particles and to a developer to be used in electrophotography. More particularly, the present invention relates to microcapsule particles having included colloidal silica therein which can be applied for said developer and to the method for preparation thereof.
2. Description of the Prior Art
In the prior art, there have been known a number of electrophotographic methods as disclosed in U.S. Pat. Nos. 2,297,691, 3,666,363 and 4,071,361. Generally speaking, the electrophotographic method comprises forming electrical latent images on a photosensitive member by utilization of a photoconductive material according to various means, then developing said latent images by use of a toner, optionally transferring the toner image onto a transfer material such as paper, and thereafter fixing the developed image by heating, pressure or solvent vapor to obtain copied products.
There have also been known various methods to visualize the electrical latent image by use of a toner.
For example, a larger number of developing methods have been known, such as the magnetic brush method as disclosed in U.S. Pat. No. 2,874,063, the cascade developing method as disclosed in the U.S. Pat. No. 2,618,522, the powder cloud method and the fur brush method as disclosed in U.S. Pat. No. 2,221,776, the liquid developing method, and others.
The toner image developed is transferred onto a transfer paper such as paper, if desired, and then fixed. As the method for fixing toner images, there have been known the method in which the toner is heated and melted by a heater or a hot roller to be fused and solidified onto the support, the method in which the binder resin in the toner is softened or dissolved by an organic solvent to be fixed onto the support, the method in which the toner is fixed by application of pressure onto the support, and others.
As the toner to be used in these methods, there have been conventionally used fine powders in which dyes or pigments are dispersed in natural or synthetic resins. Further, a third material is added for various purposes. For example, for enhancement of the developing characteristics of a developer, there may be employed flowability improvers such as colloidal silica, alumina, talc powders, etc. or materials capable of controlling charging selected from dyes or pigments as charging controlling agents. During prolonged usage, toners are subjcted to repeated frictions and are excessively charged until they are strongly adsorbed electrostatically onto carriers or the developing sleeve and cannot be peeled off by the developing electrical field, whereby charging of fresh toners may be prevented. For the purpose of releasing a part of the charges on the excessively charged toners, carbon black, fine powders of metals or fine powders of metal oxides may sometimes be added externally, or alternatively a surfactant or a polar solvent is applied by spraying. Otherwise, for preventing film formation of the toner on the photosensitive plate or adhesion of the toner onto carriers or the developing sleeve, it is also common to add externally an abrasive or a lubricant such as teflon powders, cerium oxide, vinylidene fluoride, molybdenum disulfide, etc.
Thus, a variety of substances are added into a developer for various purposes. Presence of these additives has an influence on the performance of a developer and their appropriate additive amounts are to be determined so as to preclude such an influence. However, in some cases, expected performances may not be exhibited due to deterioration or consumption out of the system of the additives during storage or prolonged usage. For example, in case of the liquid developing method, the developer comprises a toner, an inert non-polar liquid medium and a charge controlling agent. In such a developer, the charge controlling agent may sometimes be exhaustively absorbed during storage by the toner, whereby the charge controlling agent must freshly be supplied. Also, in the dry system developing method, colloidal silica which has been made hydrophobic is externally added as flowability improver, frequently and with a very high effect, but colloidol silica may sometimes receive a great shearing force in a developing vessel which causes it to be deteriorated and to make the developing characteristics of a developer instable. On the other hand, in recent years, from the viewpoint of energy saving, no pollution and instantaneous of copying, the method of pressure fixation of toner as proposed in U.S. Pat. No. 3,296,626 is attracting great attention. However, the toner which can be fixed under pressure employs a soft and tacky material, which is very high in agglomerating tendency. Therefore, it is very important to add a flowability improver, but fine particles externally added may be captured into the binder resin of the toner, whereby the fluidity of the toner becomes bad to be disadvantageously short in life. Fresh addition of a flowability improver can restore the original performance. Therefore, it may be considered to make the additive concentration in the toner supplemented higher than that in the developer at the onset of prolonged operations or to supplement the additive into the developer. However, such operations or procedures are cumbersome and therefore, under the present situation, a rigid toner binder resin is used though there are drawbacks such as curling of papers due to high fixing pressure applied, excessive lustre of papers, changes in paper thickness, large weight of fixing devices, and the like.
Colloidal silica, which has been employed in the prior art as a flowability improver, etc. in the developer for electrophotograpy, is a silicate compound bonded through ##STR1## having primary particle diameters of several millimicrons to several hundred millimicrons. It has a very bulky structure and has a very great structural viscosity in a liquid or in a resin. This is also useful in other fields than electrophotography as thickeners of paints, plastics or rubbers. Such a bulky structure can trap a large volume of air, and when colloidal silica is mixed with powders, the air trapped functions as a bearing to make powders which are fluid with low agglomerating tendency. This action is especially great in the so called hydrophobic colloidal silica prepared by alkylation of the silanol groups on the surface of colloidal silica with an alkyl halide. On the other hand, due to its bulky nature, it is very liable to fly up in the air and be difficult to settle down, thus causing troubles both in environmental hygiene and handling.